I've discovered the problem, many thanks to Talonmotorsport for his pm. It turns out that the back seat frame is twisted. The bottom rails are pretty much perfect on the build board, the left hand side of the back seat frame is pretty much perfect, but the right hand side is 10mm
further back, meaning it has put all of the other dimensions out, so i'll be spending this weekend grinding out about 10 tacks and repositioning everything else to the letter, better i noticed it now before i put any of the bracings or chassis plates on.
I'll update this thread on how i get on.
Cheers for everyones advice.